Unscrambling the Mysteries About Twisted Wire

Twisted wire has long been known to increase immunity to ambient magnetic fields as well as to decrease levels of H-field radiation from potential sources. But how effectively? This paper will answer the following questions: 1. How much are radiating magnetic fields reduced when wires are twisted? 2. How much can induced pick-up levels due to ambient H-fields be reduced by the twisting of wires? 3. How many turns per foot are most effective? 4. Is it possible to predict pick-up levels in twisted wires given a known field, drive current, and separation between source and pick-up circuits? 5. Is it possible to compute twisted wire H-field emanations if the drive current and the antenna test point are known? H-field pick-up and radiation can be reduced up to 54 dB when compared to paralleled transmission lines. However it is essential that twisted leads be grounded only at one end in order that the effect of mutual cancellation in each small loop or twist be fully realized. Both H-field pick-up and emanation levels can be accurately computed using the transfer functions shown in Figures 4, 5 and 6.